1. Field of the Invention
The present invention relates generally to a magnetoresistance effect element, a method and system for fabricating the same, and a magnetic reading system using the same. More specifically, the invention relates to a magnetoresistance effect element using a spin-valve film having an oxide layer or a nitride layer as a magnetoresistance effect improving layer for causing a specular reflection of electrons, a method and system for fabricating the same, and a magnetic reading system using the same.
2. Description of Related Art
In order to realize the enhancement of the density of hard disk drives (HDDs), a giant magnetoresistance effect (GMR) head using the magnetoresistance effect has been developed. In order to further enhance the density, a spin-valve giant magnetoresistive (SV-GMR) head is required, and it is important to further improve a rate of change in magnetoresistance (which will be hereinafter referred to as a “rate of change in MR”). The SV-GMR head means a magnetic head utilizing the giant magnetoresistance effect (GMR) based on a spin-valve (SV) film. The SV means one capable of obtaining a giant magnetoresistance by sandwiching a non-magnetic layer (which is called a “spacer layer” or a “non-magnetic intermediate layer”) between two metal ferromagnetic layers, fixing one (which is called a “pinned layer” or a “magnetization fixed layer”) of the ferromagnetic layers by a bias magnetic field or the like, and allowing the direction of magnetization of the other ferromagnetic layer (which is called a “free layer” or a “magnetization free layer”) to relatively change with respect to the pinned layer in response to the magnetic field from a recording medium (see Phys. Rev. B., Vol. 45, 806 (1992), J. Appl. Phys., Vol. 69, 4774 (1991), etc.).
The inventors have proposed a nano oxide layer specular spin-valve (NOL-SPSV) film which utilizes the specular (mirror reflection) effect to improve the rate of change in MR and which has a practical spin-valve film structure (Kamiguchi et al., “CoFe SPECULAR SPIN VALVES WITH A NANO OXIDE LAYER”, Digests of Intermag '99, DB-01, 1999). A very thin oxide film called a nano oxide layer (NOL) is inserted into a free (magnetization free) layer or a pinned (magnetization fixed) layer to allow conduction electrons to reflect on its interface to allow a spin-valve film to serve as a pseudo artificial lattice film, so that it is possible to improve the rate of change in MR. With this structure, the inventors succeeded in greatly improving the rate of change in MR of a spin-valve film.
A specular spin-valve (SPSV) film wherein an oxide is stacked on a free layer or a pinned layer to utilize the specular reflection of electrons on its interface to extend a mean free path to obtain a great magnetoresistance effect is widely noticed as a technique for realizing a recording density of 50 gigabits square inch (Gbpsi) or more. In order to use the SPSV as a practical device, there has been proposed a structure wherein an oxide layer of a few nanometers is inserted into a pinned layer or a structure wherein an oxide layer is stacked on a free layer.
However, it is very difficult to insert a NOL into a pinned layer to maintain a high specular rate while holding a magnetically sufficient strong coupling of top and bottom pinned layers via the NOL.
For example, when oxygen gas is caused to flow to form a NOL, if the oxygen exposure amount is increased at an oxidizing step of forming the NOL in order to obtain a high specular rate, there is a problem in that the magnetic coupling via the NOL is weakened whereas the specular rate is not so improved.
Inversely, if oxidation is carried out at a weak oxygen exposure amount, there is a problem in that the magnetic coupling of the top and bottom pinned layers via the NOL increases whereas a high specular rate is not obtained and the rate of change in MR is not so improved.
In addition, the simple method for causing the flow of oxygen has the merit of having a good controllability from a low amount of oxygen to a high amount of oxygen since it has only to control the flow of oxygen. However, since no energy is applied in the oxidizing process, the control range of physical properties of a NOL capable of being formed is narrow, and the control range in the process is also very narrow.
On the other hand, in order to form a NOL having a high specular rate and a small influence of oxygen diffusion by a stable oxide, there is also the idea that a high energy process is used without using the above described simple oxidation based on the flow of oxygen.
For example, there is a method for making the plasma of oxygen as an ion beam to irradiate the surface of a film, which is to be oxidized, with the ion beam. This technique is effective in a high energy process. However, since the surface of a sample is exposed to suspended oxygen before oxygen plasma is fired (produced), there is a problem in that an oxide film is formed by simple oxygen gas, so that the same layer as a natural oxide film is formed in the portion of an initially grown NOL. The initial NOL layer produced at a low energy and at a low reactivity decreases ΔGs which serves as an index of a rate of change in MR.
This is the same in the case of plasma oxidation and radical oxidation. Although it is possible to carry out a high energy oxidation by plasma energy, there is a problem in that the portion of the initially grown NOL is always influenced by a natural oxide film based on suspended oxygen.
As described above, in the case of the natural oxidizing method, the controllability of the oxygen exposure amount from the low flow amount of oxygen to the high flow amount of oxygen is relatively excellent, whereas it is not possible to obtain a high specular rate since no energy is applied during oxidation. On the other hand, the IBO, the radical oxidation and the plasma oxidation are effective in a high energy oxidation, whereas there are problems in that it is not sufficiently control the amount of oxygen so that it is not possible to control a low amount of oxygen and that a natural oxide film is also formed as an initially grown NOL.
The inventors concluded that it is very difficult to obtain a stable oxide by a natural oxidation by which no energy is supplied, in order to obtain a high specular rate and thermally stabilize oxygen in a NOL. However, in the conventional high energy process, it is difficult to control a low amount of oxygen, and the initially grown NOL always becomes a natural oxide film.
It has been reported that magnetoresistance (MR) improving results are obtained if a structure wherein an oxide of Ta, Fe, Ni or Al is stacked directly on a magnetic layer of a free layer is used as an electron reflective layer on the side of the free layer. It also has been reported a SV film using a spin filter (SF) structure can also obtain MR improving effects due to an electron reflective layer.
The SF is a structure wherein a high conductive layer is stacked on a free layer, a structure which is effective in a bias point design in the output of a magnetic head, and a structure which is necessary for a magnetic head corresponding to a high recording density. It has been reported that a structure wherein a Ta oxide is formed on a Cu high conductive layer in this SF structure has MR improving effects. A structure wherein an NiO film having a thickness of 10 nm is formed on a Cu high conductive layer also has been reported.
However, oxides of Ta, Fe and Al among the above described oxides are multi-phases in which an amorphous or several kinds of crystalline structures exist. In such a case, the electron reflective effect decreases, so that it is not possible to sufficiently increase the MR. In addition, with respect to thermal stability, these fine structures are easy to cause diffusion and transformation to cause the deterioration of MR characteristics. Moreover, the quality of NiO is not so sufficiently good unless its thickness is about more than 10 nm, that the increase of the MR based on the electron reflective effect is small.
With the improvement of recording density, it has been requested to decrease the whole thickness of an SV film. From this point of view, it is difficult to use the above described electron reflective layer having a thickness reaching 10 nm for a device.
Moreover, when a magnetic head including an SV film is prepared, an alumina layer is formed above and below the SV film as an insulating layer. However, the alumina layer is simply formed on an oxide of amorphous, oxygen diffuses into the alumina layer from the oxide to produce portions in which a partially oxygen deficiency state is caused in an electron reflective layer and the oxide is reduced to a metal.
In general, it is required to stack a Ta metal on the uppermost portion of a few nm. However, also in this case, O2 is absorbed into the Ta metal to reduce a part of an electron reflective layer, so that an oxygen deficiency portion and a reduced metal portion appear in the electron reflective layer. These greatly cause to reduce the electron reflective effect, cause deterioration of output, and cause deterioration of long-term thermal stability.
As described above, in the conventional structure wherein the oxide electron reflective layer is stacked on the side of the free layer, the MR is improved, whereas there are many problems as a device which is put to practical use, so that it is difficult to put the device to practical use.